Cambodian-Vietnamese War

The Cambodian-Vietnamese War (30 April 1977-23 October 1991) was fought between Vietnam and the puppet People's Republic of Kampuchea in Cambodia and the United States and China-backed Khmer Rouge guerrillas and its allied rebel groups. The war began with isolated clashes between Vietnam and Democratic Kampuchea along their land and maritime borders in 1975 and 1976, with the Khmer Rouge government of Kampuchea purging Vietnamese-trained officers from the Cambodian army out of fear that Vietnam would create a communist federation in Southeast Asia. On 30 April 1977, the Kampuchean army launched a major military attack on Vietnam, leading to a retaliatory Vietnamese strike later that year. China attempted to negotiate a peace as fighting continued into 1978, but 150,000 People's Army of Vietnam troops invaded Cambodia on 25 December 1978, launching a massive invasion of the country. On 8 January 1979, the Khmer Rouge government was overthrown and replaced by the pro-Vietnamese "People's Republic of Kampuchea", ending the Cambodian Genocide and forcing Pol Pot and the Khmer Rouge to wage guerrilla war against the Vietnamese. China intervened military on behalf of the Khmer Rouge, beginning the 1979 Sino-Vietnamese War, which ended in stalemate. Both China and the United States sent military aid to the genocidal Khmer Rouge guerrillas after discovering that the Soviet Union was arming the Vietnamese, backing the opposing side in the war.

Vietnamese troops occupied Cambodia to assist the PRK with fighting against the Khmer Rouge, but resistance movements grew larger and larger as the Vietnamese military presence remained constant. The fighting spilled over into Thailand as Khmer Rouge guerrillas launched cross-border attacks on Cambodia from the Thai side of the border, and border warfare occasionally broke out between the Thai military and the Vietnamese. In September 1989, under immense political and economic pressure, Vietnam agreed to withdraw its troops from Cambodia, and Cambodia would be governed by the United Nations until the mediator and centrist Hun Sen became the Prime Minister of Cambodia. The war left 25,300 Vietnamese dead and 30,000 wounded, while the Cambodians lost 50,000+ dead; 100,000+ civilians died, excluding famine deaths.